This paper discusses using hydraulic models for system-specific studies associated with Initial Distribution System Evaluations (IDSEs), which will be required by the US Environmental Protection Agency's (USEPA) Stage 2 Disinfectants and Disinfection Byproducts Rule. The use of calibrated hydraulic models is one of two suggested approaches for completing system-specific studies. The paper reviews topics that include: minimum model requirements concerning skeletonization, or level of detail; water consumption, including diurnal patterns; and, calibration. Tracer studies are discussed, as well as the USEPA's recommendation to compare predicted residence times with measured disinfectant concentrations. The paper also describes how residence time, source influence zones and mixing zones are considered when selecting preliminary monitoring sites.